International Publication No. 2010/035717 discloses, in a spark plug in which an insulator is retained by a metal shell, a technology of making a portion between the metal shell and the insulator airtight by using a metallic packing. When a load that the metal shell and the insulator apply to the packing is increased, the airtightness is increased, whereas, when the excessively deformed packing strongly compresses the insulator, the insulator breaks. In the technology of International Publication No. 2010/035717, the shape of a gap between the metal shell and the insulator is adjusted to suppress excessive deformation of the packing, so that airtightness is ensured while suppressing occurrence of cracking of the insulator.
However, in the above-described existing technology, there is a demand for increasing the airtightness between the metal shell and the insulator without excessively increasing the load when retaining the insulator by the metal shell.